Optimize the build size

[0xdevalias]

Currently this builds to ~200mb, with most of the size being in the apt-get layers (~170mb) and git clone (~70mb)

⇒  docker history devalias/oxml_xxe
IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
61c747297e90        5 minutes ago       /bin/sh -c #(nop)  CMD ["-o" "0.0.0.0"]         0B
42e88a2040e4        5 minutes ago       /bin/sh -c #(nop)  ENTRYPOINT ["ruby" "ser...   0B
0d1295d90b0c        6 weeks ago         /bin/sh -c #(nop)  EXPOSE 4567/tcp              0B
2e5eb9441a60        6 weeks ago         /bin/sh -c #(nop) WORKDIR /oxml_xxe             0B
336d7a46f3d6        6 weeks ago         /bin/sh -c git clone --depth=1 https://git...   70MB
e7c8f27ae7ef        6 weeks ago         /bin/sh -c apt-get update &&     apt-get i...   172MB
c53310a030bd        6 weeks ago         /bin/sh -c #(nop)  CMD ["irb"]                  0B
<missing>           6 weeks ago         /bin/sh -c mkdir -p "$GEM_HOME" "$BUNDLE_B...   0B
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV PATH=/usr/local/bun...   0B
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV BUNDLE_PATH=/usr/lo...   0B
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV GEM_HOME=/usr/local...   0B
<missing>           6 weeks ago         /bin/sh -c gem install bundler --version "...   1.6MB
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV BUNDLER_VERSION=1.15.4   0B
<missing>           6 weeks ago         /bin/sh -c set -ex   && buildDeps='   auto...   121MB
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV RUBYGEMS_VERSION=2....   0B
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV RUBY_DOWNLOAD_SHA25...   0B
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV RUBY_VERSION=2.3.5       0B
<missing>           6 weeks ago         /bin/sh -c #(nop)  ENV RUBY_MAJOR=2.3           0B
<missing>           6 weeks ago         /bin/sh -c mkdir -p /usr/local/etc  && {  ...   45B
<missing>           6 weeks ago         /bin/sh -c apt-get update  && apt-get inst...   30.7MB
<missing>           6 weeks ago         /bin/sh -c #(nop)  CMD ["bash"]                 0B
<missing>           6 weeks ago         /bin/sh -c #(nop) ADD file:d7333b3e0bc6479...   123MB

See: https://hub.docker.com/r/devalias/oxml_xxe/

#1 may also help/solve this